Conventionally, the speed change stage is determined based on the vehicle speed and throttle valve opening degree of the engine (acceleration pedal operation amount) for the speed change control device of an automatic transmission for vehicle. Another method for determining the speed change stage is known. In this method, the vehicle weight, which is variable depending on the number of passenger or the loaded condition of the vehicles is calculated based on the vehicle acceleration and the driving force applied to the vehicle and such calculated vehicle weight is used for determining the speed change stage of the automatic transmission control device to improve the engine brake performance during the vehicle being running on a down slope or to improve the acceleration performance during the vehicle being running on an up slope.
Such weight determination method is disclosed in a Japanese Patent Laid-Open publication No. 2000-213981. This conventional device calculates the vehicle weight based on filtered acceleration and driving force for removing any road inclination factors by filtering the detected acceleration and driving force using high-pass filter. In this method, filtering is processed at the point where the influence of the road inclination appears as the low frequency component.
However, in the above structure, due to the fact that the acceleration is largely changed or fluctuated by the torsion of drive train of the vehicle and the torsion or vibration of the suspension system of the vehicle, which problems are not yet sufficiently solved, the determination accuracy is not sufficiently achieved.